Field of Disclosure
Embodiments described herein generally relate to quantify skin prior to hydraulic fracturing of low and tight reservoirs. Specifically, embodiments discussed herein are directed towards using a correlation to quantify the skin in pre-analysis of hydraulic fracturing study which affect the overall deliverability of a reservoir and to plan hydraulic fracturing design accordingly.
Description of the Related Art
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Hydraulic fracturing is frequently practiced to stimulate unconventional and tight reservoirs. The technique increases the deliverability of a reservoir by inducing a reduction in pressure drawn downwards and exposing more reservoir contact. Further, fracturing can also help break through severe formation damage and can make low permeability and tight reservoirs economically viable.
Current economies emphasize the need for optimizing the production of gas from tight reservoirs. Such optimization is important considering that the costs of drilling new wells and operating existing wells are high by historical standards, largely because of the extreme depths to which new producing wells must be drilled and because of other physical barriers to discovering and exploiting reservoirs. These high economic stakes demand operators to devote substantial resources toward effective management of reservoirs.
Prior to executing hydraulic fracturing, the feasibility of the fracturing operation can be assessed by analyzing the effect of skin on the overall deliverability of the system. There are two ways to quantify the skin prior to execution of the hydraulic fracturing: either by using analytical models or numerically simulating the whole scheme for evaluating this effect. However, such analytical models are either based on simple assumptions which could lead to wrong estimation of skin, or the numerical simulations are too complex to solve, and their processing times are unacceptable. After, the execution of the job, the true value of skin can be calculated by conducting the well test analysis. The focus of the work conducted thus far has been on the building of pressure transient analysis or assessment of characteristics of an existing fracture and its associated conductivity, and the determination and improvement of productivity of an optimized fracture design. Outcomes of these investigations are presented in the form of plotted graphs, or correlations that outline the productivity, skin factor or effective well-bore radius.
The focus of the work conducted thus far has been on building of pressure transient analysis or assessment of characteristics of an existing fracture and its associated conductivity and the determination and improvement of productivity of an optimized fracture design. Outcomes of these investigations are presented in the form of plotted graphs, or correlations that outline the productivity, skin factor or effective well-bore radius.
For example, the work conducted by McGuire and Sikora studies the effect of finite conductivity of a vertical fracture on the productivity of wells in a pseudo steady state with the help of an electric analogue computer. The curves obtained by this study are still one of most broadly used reference diagnostics plots for productivity forecasting. The McGuire and Sikora curves validate the productivity of hydraulic fracturing as a function of fracture length (penetration) and relative conductivity.
Prats devised an analytical model for prediction of pseudo steady state behavior in a system of finite conductivity vertical fracture. In this work, Prats introduced the notion of effective well bore radius and the realization that there is an optimum length-width ratio (dimensionless conductivity) for a given fracture volume that maximizes the productivity. Prats concluded that smaller values of effective wellbore radius correspond to less effective fractures. Prats established that the pressure profile in the fracture system is a function of fracture half-length and relative capacity (a), and is defined as:
      a    =                  π        *        K        *                  X          f                            2        *        W        *                  K          f                                r              w        D            ′        =                  r        W        ′                    X        f            
In another work, Cinco-ley and Samaniego outlined a new technique for evaluating pressure transient analysis for vertical finite-conductivity fracture. The major contributions of this work were: 1) the introduction of pseudo skin function and 2) the presentation of curves for dimensionless effective wellbore radius versus fracture conductivity and damaged fracture. The concept of equivalent skin effect for pseudo radial and pseudo steady state flow varying with fracture conductivity were obtained from their analysis.
      C          f      D        =                    K        f            ×      w              K      ×              X        f            
Economides and Valko determined the pseudo steady state productivity index of a fractured well and obtained some charts for optimum design of fractured wells.
      J          D      pss        =      1                  Ln        ⁢                              r            e                                X            f                              -      0.75      +              Ln        ⁢                              X            f                                r            w                              +              S        f            
Further, Meyer and Jacot introduced dimensionless productivity index and equation for effective wellbore radius. Meyer and Jacot also introduced a simple equation for estimating pseudo fracture skin when fracture length is negligible as compared to reservoir length.
      r    w    ′    =            X      f                      π                  C                      f            D                              +      2      
In the formulas above the variables represent the following:
CfD=fracture conductivity.
h=formation thickness
k=absolute reservoir permeability
kf=absolute fracture permeability
P=pressure
q=flow rate
r=radius
S=skin factor
wf=fracture width
Xf=Half length of the fracture
μ=viscosity
ρ=density
JDpss=Dimensionless Productivity Index
Subscripts represent the following
d=damage
D=dimensionless
e=external as in re
i=an index
j=an index
f=fracture
w=well-bore
Skin is a valuable tool for determining the performance of hydraulic fracturing. Proper quantification of the effect of hydraulic fracturing in a simulation model is critical due to the difference of fluid flow behavior between the fracture and porous media. A conventional but costly approach to cater to this effect is to locally refine grid cells around the well bore for capturing the abrupt changes in flow parameters in this vicinity. The exercise is inevitable since normal grid size employed for rest of the system cannot model fractures with average widths of 1 cm. A cost effective and easier solution is introducing skin, which can assist reservoir engineers to predict the enhanced productivity attributed to fractures. Accordingly, there is a requirement for a cost-effective model that determines skin factor of the reservoir using correlation within relationship with other critical parameters that define a hydraulically fractured reservoir.